The present invention relates to power generation systems and refrigeration systems, and more specifically relates to a system with evaporator employing mixed refrigerants for power generation systems and refrigeration systems.
The Organic Rankine Cycle (ORC) is commonly used as a power generation system for low temperature resources such as geothermal, solar thermal, biomass, and waste heat recovery. The primary components of an ORC system include an expansion device, a condenser, an evaporator/gas heater, and a motive pump. Traditionally Organic Rankine Cycle systems employ flooded evaporators, which use a shell and tube construction in order to evaporate a pool of liquid to produce superheated vapor. In typical flooded evaporators, a resource, such as hot water or hot fluid, flows through tubes. In less conventional systems, a hot gas flows through smoke tubes. The resource facilitates heat exchange between a pool of liquid, usually a working fluid comprised of a refrigerant, and the surface of the tubes to evaporate the liquid, resulting in superheated vapor. To continue the cycle, the superheated vapor exits the evaporator, expands in a turbine, spinning a generator, which then produces electricity. Low pressure and low temperature vapor exits the turbine and flows through a condenser where a cooler medium, such as air or water, condenses the vapor into liquid in a condenser. Liquid from the condenser is then pumped back into the pool of the flooded evaporator to repeat the cycle.
Flooded evaporators are disadvantageous for power generation cycles in terms of cost, environmental impact, footprint, and efficiency. Flooded evaporators require a significant amount of refrigerant charge to cover enough tubes to maintain sufficient heat transfer in order to evaporate the refrigerant liquid. In order to control the degree of superheat in order to maintain optimal turbine and system performance, a predetermined number of tubes remain unwetted in order to superheat the vapor being generated in the evaporator. The number of tubes that need to remain wetted is still quite significant, requiring a significant amount of refrigerant charge. Using a flooded evaporator, particularly for systems that utilize hydrofluorocarbons or other relevant working fluids, poses a significant cost concern due to the significant initial refrigerant charge, as well as the charge needed for maintenance and replenishment.
The heat transfer penalty associated with the use of non-azeotropic mixed refrigerants in conventional flooded evaporators significantly reduces the amount of power that can be generated by such a system. Some non-azeotropic mixed refrigerants may exhibit lower heat transfer coefficient due to a reduced interfacial temperature between the liquid and vapor phases. This reduced interfacial temperature gives rise to heat and mass transfer resistances.